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Heterodyne Detector for Measuring the Characteristic of Elliptically Polarized Microwaves

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Heterodyne Detector for Measuring the Characteristic of Elliptically Polarized Microwaves Downloaded from orbit.dtu.dk on: Oct 02, 2021 Heterodyne detector for measuring the characteristic of elliptically polarized microwaves Leipold, Frank; Nielsen, Stefan Kragh; Michelsen, Susanne Published in: Review of Scientific Instruments Link to article, DOI: 10.1063/1.2937651 Publication date: 2008 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Leipold, F., Nielsen, S. K., & Michelsen, S. (2008). Heterodyne detector for measuring the characteristic of elliptically polarized microwaves. Review of Scientific Instruments, 79(6), 065103. https://doi.org/10.1063/1.2937651 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. REVIEW OF SCIENTIFIC INSTRUMENTS 79, 065103 ͑2008͒ Heterodyne detector for measuring the characteristic of elliptically polarized microwaves Frank Leipold, Stefan Nielsen, and Susanne Michelsen Association EURATOM, Risø National Laboratory, Technical University of Denmark, OPL-128, Frederiksborgvej 399, 4000 Roskilde, Denmark ͑Received 28 January 2008; accepted 6 May 2008; published online 4 June 2008͒ In the present paper, a device is introduced, which is capable of determining the three characteristic parameters of elliptically polarized light ͑ellipticity, angle of ellipticity, and direction of rotation͒ for microwave radiation at a frequency of 110 GHz. The device consists of two perpendicular orientated pickup waveguides. A heterodyne technique mixes the microwave frequency down to frequencies on the order of 200 MHz. An oscilloscope is used to determine the relative amplitudes of the electrical fields and the phase shift in between, from which the three characteristic parameters can be calculated. Results from measured and calculated wave characteristics of an elliptically polarized 110 GHz microwave beam for plasma heating launched into the TEXTOR-tokamak experiment are presented. Measurement and calculation are in good agreement. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2937651͔ INTRODUCTION characteristics obtained by polarizer plates in the transmis- Elliptically polarized microwaves in the range above sion line can be calculated from the orientation of the polar- ͑ ͒ 100 GHz play an important role for plasma heating, current izer plates when their characteristics are known . However, drive, and scattering of radiation in tokamak plasma it is desirable to verify the calculated results by measure- experiments.1,2 High power microwave sources ͑gyrotrons͒ ments. are used for these applications. A gyrotron generates linearly Elliptically polarized light is obtained, when two perpen- polarized waves. In order to transform a linearly polarized dicularly orientated linearly polarized waves with a phase wave into an elliptically polarized wave, polarizer plates are shift are superimposed. The propagation directions of both employed.3,4 A polarizer plate is a mirror with parallel waves is the z direction and Ex and Ey are the electrical field grooves on the surface. In a simplified picture, an electrical components in the x and y directions, respectively. Figure 1 ͑ ͒ field component which is oriented parallel to the grooves is shows an example of two waves Ex and Ey with a phase / reflected on top of the grooves. The electrical field compo- shift of 50°. The ratio of the amplitudes Ey Ex is 0.7. Figure nent, which is oriented perpendicular to the grooves is re- 2 shows the projection of the electrical field in the x-y plane. flected at the bottom of the grooves. This causes a phase shift The strength of the electrical field is represented by the dis- with respect to the electrical field component reflected on top tance between the origin and the boundary of the ellipse. The of the grooves. A detailed description of the diffraction of electrical field vector rotates, as described in the picture. electromagnetic waves with perfectly electrical conducting The characteristic of elliptically polarized light can be rectangular grooved grating is described in Ref. 5. The phase described by three parameters: ellipticity ͑␧͒, angle of ellip- 8–10 shift depends on the design of the polarizer plate. The groove ticity ͑⌰͒, and direction of rotation ͑␳͒. ␧ is the ratio of depths and width are dependent on the used frequency and the minor axis to the major axis of the ellipse describing the the desired phase shift. The superposition of both electrical electrical field in the x-y plane. It varies between 0 ͑linearly field components provides, in general, elliptically polarized polarized light͒ and 1 ͑circularly polarized light͒. ⌰ is the waves. angle between the major axis of the ellipse and the x axis in Characterization of elliptically polarized microwaves in turning direction toward the y axis ͑mathematically positive the frequency range of 110 GHz is important, e.g., for the direction͒. It varies between 0° and 180°. ␳ can either be diagnostic technique of collective Thomson scattering.6,7 left-hand polarized or right-hand polarized. The electrical This technique uses a probing microwave beam launched field vector of a right-hand polarized wave propagating in the into the plasma and detects scattered light leaving the positive z direction in a right handed coordinate system ro- plasma. Probing beam and scattered signal are generally el- tates in the mathematically negative direction when projected liptically polarized. In the case of the probing beam, the re- into the x-y plane. The transformation from the parameters, quired elliptically polarized light is obtained by transforming Ex, Ey, and phase shift between Ex and Ey to the character- linearly polarized waves from a gyrotron by means of a set istic parameters ⌰, ␧, and ␳ is described in detail in Ref. 8. of two polarizer plates. The elliptically polarized scattered In the example above ͑Figs. 1 and 2͒, the angle of ellipticity signal leaving the plasma is transformed by means of polar- ⌰ is 30.2°, the ellipticity ␧ is 0.42, and the rotation ␳ is izer plates into linearly polarized light which can be accepted right-hand polarized. by the receiver. Theoretically, the elliptically polarized wave ␧ and ⌰ of elliptically polarized waves can be obtained 0034-6748/2008/79͑6͒/065103/4/$23.0079, 065103-1 © 2008 American Institute of Physics Downloaded 13 Aug 2009 to 192.38.67.112. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp 065103-2 Leipold, Nielsen, and Michelsen Rev. Sci. Instrum. 79, 065103 ͑2008͒ FIG. 3. Schematic of the detector for elliptically polarized light. second step, the y component of the electrical field is phase shifted by 90° degrees, and again, sum and difference signals are recorded. These four values allow the characterization of the elliptically polarized waves. The present article reports about a two-channel hetero- dyne detection system which is built in order to fully char- ͑ ͒ FIG. 1. The relative electrical field strength in the directions of x Ex and y acterize elliptically polarized waves instantaneously without ͑E ͒ E E y . In this example, y leads x by 50°. the use of movable mechanical parts. Principles of hetero- dyne techniques can be found in Refs. 13 and 14. by detecting the light intensity in various orientations of a detector diode in the x-y plane. This technique does not re- quire a complex system. However, since the phase difference DEVICE SETUP of the measured electrical field in various directions cannot The intensity of microwave radiation can easily be mea- be detected, the rotation of the elliptically polarized wave sured by detector diodes or Schottky diodes, but this tech- cannot be obtained in this approach. Furthermore, several nique does not provide information about the phase between measurements in various orientations of the detector are re- the electrical field components detected in the two channels. quired, which is a time consuming procedure when a com- In order to detect the phase difference between the fields in plete mapping of a transmission line containing several po- the two channels, a more sophisticated technique is required. larizers is needed. The authors in Ref. 11 used a rotating For the description and operation of this device, which is birefringent plate in front of a detector in order to character- sketched in Fig. 3, a right-handed coordinate system is con- ize an elliptically polarized wave. This technique provides all sidered, in which the wave propagates in the positive z di- three parameters of the elliptically polarized wave. The au- rection. The electrical field of the wave is perpendicular to thors in Ref. 12 introduced a method for characterization of the axis of propagation and has components in the x and y an elliptically polarized wave, in which the detected x and y directions which are referred to as Ex and Ey, respectively. components of the electrical field are added and subtracted in The axis of the detector is aligned parallel to the z axis. The a “Magic T.” Sum and difference signals are recorded. In a detector has two channels, which can detect the electrical field in two orthogonal orientations in the x-y plane simulta- neously. In order to obtain the three characteristic parameters of the elliptically polarized wave, the amplitudes of the sig- nal in each channel and the phase between the two measured signals need to be known.
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